Using immunohistochemical techniques in combination with retrograde tracing, we have extended our earlier findings in the paraventricular n. (PVN) to show that adrenalectomy (ADX) increases vasopressin (VP) immunostaining in the PVN and under these conditions VP and corticotropin releasing factor (CRF) coexist in single neurons in the brain. In addition, treatment of animals with an inhibitor which reduces brain epinephrine concentrations increased CRF staining suggesting that the innervating transmitter epinephrine may play a role in peptide expression. In addition we have established that it is an epinephrine-neuropeptide Y coexistent projection which innervates CRF neurons in the PVN. Finally, in addition to VP, in some cases cholecystokinin (CCK) coexists with CRF in the PVN. In a second series of studies we have extended our radioimmunocytochemical studies on the distribution of substance P and leucine-enkephalin in the rat brain. We find that these two putative transmitters now relate very well except in a few specific brain nuclei.